The fuel tanks on an aircraft are a major portion of the weight of the whole craft. So make engines more efficient, and you can take less fuel, which means the plane is lighter, so you use even less fuel and can take even less, etc. As you reduce the amount of fuel needed to go a certain distance you can go further and convert two-leg flights to single hops, which saves all sorts of other costs.

The fuel tank of a semi truck is not an important proportion of the weight of the whole vehicle, so it does matter, but not so much.

Wind resistance goes up by the cube of the velocity, it would be a much larger savings on an airplane.

Still even at interstate speeds a 5% savings pays for itself pretty quickly.